1. Field of the Invention
The present invention is related to thermal monitoring and stress calculation for a containing system and, more particularly, to a method and apparatus for calculating stress caused by time dependent, spatially distributed thermal loads in a system such as a steam supply system in a pressurized water nuclear reactor.
2. Description of the Related Art
Cooling systems typically include a piping network or system of pipes including valves, headers and other junctions. Temperature fluctuations of the fluid contained by such systems produce thermal loads which cause stress on the system. In systems which maintain the pressure boundary in nuclear reactors used for electrical power generation, monitoring systems are used to detect thermal loads on the system. U.S. Pat. Application Ser. No. 07/018,379 filed Feb. 24, 1987, is one example of a system for monitoring thermal loads and calculating the stresses caused by the thermal loads. The system disclosed in the '379 Application is particularly directed to monitoring changes in thermal loads on an entire pipe or other component and calculating fatigue accumulation from the cyclical changes in thermal loads. Other conventional thermal load monitoring systems are more simplistic and generally do not go beyond recording temperature at a single point for each pipe or other component which is monitored.
Under certain conditions of low or no flow, the fluid in a containment system can become stratified with sharply different temperature levels. While the stratification itself creates an abnormally high thermal bending load due to the sharp change in temperature at the interface between the regions of different temperatures, the stress is increased when the interface between layers oscillates rapidly in a small band about some mean level. Such fluctuation is termed thermal striping. The stress on the system also changes when the global interface level varies up and down the cross section of the pipe. When such changes in the location of the interface are repeated, the load is considered thermocyclic.
The stress on the system also changes when the global interface level varies up and down the cross-section of the pipe. None of the existing monitoring systems make any attempt to detect the existence of stratification layers in containment systems or to calculate the stress caused by such stratification.